The body requires a certain volume of air to be inhaled and exhaled to maintain the correct levels of oxygen and carbon dioxide within the tissues. Tissue damage, which leads eventually to death, occurs if the level of oxygen becomes too low or the amount of carbon dioxide becomes too high. The body is therefore critically dependent on breathing to maintain life. In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. A medical ventilator moves breathable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently. Ventilators are chiefly used in intensive care medicine and emergency medicine (as standalone units) and in anesthesia (as a component of an anesthesia machine).